1. Field of the Invention
The invention is related generally to the use of resistivity measurements for evaluation of earth formations having bedding in which the dip varies away from a borehole.
2. Background of the Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that when properly interpreted are diagnostic of the petrophysical properties of the formation and the fluids therein.
The physical principles of electromagnetic induction well logging are described, for example, in, H. G. Doll, Introduction to Induction Logging and Application to Logging of Wells Drilled with Oil Based Mud, Journal of Petroleum Technology, vol. 1, p. 148, Society of Petroleum Engineers, Richardson Tex. (1949). Many improvements and modifications to electromagnetic induction resistivity instruments have been devised since publication of the Doll reference, supra. Examples of such modifications and improvements can be found, for example, in U.S. Pat. No. 4,837,517 to Barber et al., U.S. Pat. No. 5,157,605 issued to Chandler et al, and U.S. Pat. No. 5,452,761 issued to Beard et al.
In recent years, increasing use has been made of multi-component resistivity measurements that are responsive to vertical and horizontal resistivities (or, equivalently, conductivities) of anisotropic formations. The terms “horizontal” and “vertical” as commonly used and as used in this document relate to directions that are parallel to and perpendicular to bedding, and the resistivities in these directions are commonly the minimum and maximum in an anisotropic formation.
U.S. Pat. No. 5,999,883 issued to Gupta et al, the contents of which are fully incorporated here by reference, discloses a method for determination of the horizontal and vertical conductivity of anisotropic earth formations. Electromagnetic induction signals induced by induction transmitters oriented along three mutually orthogonal axes are measured. One of the mutually orthogonal axes is substantially parallel to a logging instrument axis. The electromagnetic induction signals are measured using first receivers each having a magnetic moment parallel to one of the orthogonal axes and using second receivers each having a magnetic moment perpendicular to a one of the orthogonal axes which is also perpendicular to the instrument axis. A relative angle of rotation of the perpendicular one of the orthogonal axes is calculated from the receiver signals measured perpendicular to the instrument axis. An intermediate measurement tensor is calculated by rotating magnitudes of the receiver signals through a negative of the angle of rotation. A relative angle of inclination of one of the orthogonal axes which is parallel to the axis of the instrument is calculated, from the rotated magnitudes, with respect to a direction of the vertical conductivity. The rotated magnitudes are rotated through a negative of the angle of inclination. Horizontal conductivity is calculated from the magnitudes of the receiver signals after the second step of rotation. An anisotropy parameter is calculated from the receiver signal magnitudes after the second step of rotation. Vertical conductivity is calculated from the horizontal conductivity and the anisotropy parameter.
U.S. Pat. No. 6,466,872 to Kriegshauser et al. having the same assignee as the present application and the contents of which are fully incorporated herein by reference discloses use of a multi-component logging tool (the 3DEX™ tool of Baker Hughes Incorporated) for determination of anisotropic resistivity parameters of a laminated reservoir. As would be known to those versed in the art, such a laminated reservoir that has layers of different resistivities exhibits transverse isotropy even if the layers themselves are isotropic. Such a multicomponent logging tool has azimuthal sensitivity. Kriegshauser discloses a method of analyzing data from a multicomponent logging tool to determine water saturations of the sand and shale fractions of the reservoir. The model used in Kriegshauser assumes that the anisotropy axis is normal to the bedding plane. Similar models have been assumed in, for example, in U.S. Pat. No. 6,618,676 to Kriegshauser et al., and in U.S. Pat. No. 6,643,589 to Zhang et al.
The 3DEX™ tool has a depth of investigation in the formation that is typically several meters and correspond to large-scale dip and azimuth. In contrast, the dips and azimuths from imaging devices are derived from the property (e.g., resistivity) boundaries of formation beds or laminations. When the beds or laminations are within the resolution of the imaging devices, the dips and azimuths are reliably determined. In contrast, the 3DEX™ measurements are sensitive to the orientation of the formation conductivity tensor. The measurements allow us to accurately determine the dips and azimuths in the absence of bed boundaries, provided there exists measurable formation anisotropy. Hence, in many instances the imaging-derived dips and azimuths may be quite different from the 3DEX™-derived ones. A typical example would be in a thick anisotropic shale layer where the imaging tools may not provide reliable dips and azimuths but the 3DEX™ tool will.
Moreover, the different depths of investigation (DOI) and different vertical resolution of the 3DEX™ measurement and the conventional borehole imaging logs will in some circumstances result in different dips and azimuths. The borehole imaging tools usually have DOIs less than a few centimeters, whereas the 3DEX™ measurement reads meters into the formation. Therefore, the two measurements will read the same angles if the angles do not change significantly from the borehole. When formation angles change laterally, it must be understood how the measurement “averaging” affects the angle data derived from 3DEX™ tool measurements.
The purpose of the present invention is to identify and use multicomponent measurements to characterize geologic formations away from the borehole and/or to compare the results of this characterization with borehole imaging logs.